Any ideas on how they messed up the magazine design for the 10 round versions?
With the variety of M&P pistols I own, I have generally had very good experiences using whatever the "standard" magazine designed for the pistol. The only bad magazines I ever had were some LEO trade in .40 magazines with a lot of wear, and those were all fixed with new Wolff springs.
It seems surprising on the 10 round versions because S&W generally seems to be able to make reliable magazines that don't cost a fortune.